Flooring over a cavity

[Grinning Grandad]

We are having a new floor fitted next week and in preparation for the screed I am removing the carpet etc so they have a blank canvas. Under the carpet, in front of the patio doors there was some pieces of laminate flooring screwed down. I removed these to reveal the cavity between the outside and inside walls. It looks as though there used to be a window in the wall and they simply knocked out the walls under the window to fit patio doors and covered the cavity with laminate. Is there something I can put inside the cavity which will allow them to screed over it to provide a level surface?

 

[Tigercubrider]

That looks remarkably like a cheap tarp used as dpc?